Amelioration strategies for saline soils: a review

LAND DEGRADATION + DEVELOPMENT

Land De`rad[ Develop[ 00] 490Ð410 "1999#

Copyright Þ 1999 John Wiley + Sons\ Ltd[

AMELIORATION STRATEGIES FOR SALINE SOILS] A REVIEW

M[ QADIR\0� A[ GHAFOOR1 AND G[ MURTAZA1

0 Institut fu�r P~anzenerna�hrun`\ Giessen\ Germany1 Department of Soil Science\ University of A`riculture\ Faisalabad\ Pakistan

Received 0 December 0888^ Accepted 05 January 1999

ABSTRACT

Soil salinization is one of the major causes of declining agricultural productivity in many arid and semiarid regions ofthe world[ Excessive salt concentrations in soils\ in most cases\ cannot be reduced with time by routine irrigation andcrop management practices[ Such situations demand soil amelioration[ Various means used to ameliorate saline soilsinclude] "a# movement of excess soluble salts from upper to lower soil depths via leaching\ which may be accomplishedby continuous ponding\ intermittent ponding\ or sprinkling^ "b# surface ~ushing of salts from soils that contain saltcrusts at the surface\ a shallow watertable\ or a highly impermeable pro_le^ "c# biological reduction of salts by harvestof high!salt accumulating aerial plant parts\ in areas with negligible irrigation water or rainfall available for leachingând "d# amelioration of saline soils under cropping and leaching[ Among these methods\ cropping in conjunction withleaching has been found as the most successful and sustainable way to ameliorate saline soils[ Cropping during leachingor between leachings causes an increase in salt!leaching e.ciency because a decrease in soil water content occurs underunsaturated water ~ow conditions with a concurrent decrease in large pore bypass and drainage volume[ Consequently\anaerobic conditions in soil may occur during leaching that can a}ect crop growth[ Thus\ in addition to the existingsalt!tolerant crop genotypes\ research is needed to seek out or develop genotypes with increased tolerances to salinityand hypoxia[ Since salt leaching is interacted by many factors\ evaluation of the traditional concepts such as the leachingrequirement "LR#\ the leaching fraction "LF# and the salt balance index "SBI# demands incorporation of a rapid\ e.cientand economical way of monitoring changes in soil salinity during amelioration[ Besides this\ numerous models that havebeen developed for simulating movement and reactions of salts in soils need evaluation under actual _eld conditions[Copyright Þ 1999 John Wiley + Sons\ Ltd[

KEY WORDS] salt!a}ected soils^ leaching requirement^ surface ~ushing^ soil reclamation^ salt!tolerant crops^ salt balance^ salinityappraisal^ salt uptake

INTRODUCTION

Excess of salts in a soil can bring drastic changes in some of the soil|s physical and chemical propertiesresulting in the development of an environment unsuitable for growth of most crops[ Soils having salts inthe solution phase and:or sodium ions "Na¦# on the cation exchange sites exceeding the speci_ed limits arecalled salt!a}ected soils[ Major cations in salt!a}ected soils are Na¦\ calcium "Ca1¦#\ magnesium "Mg1¦#\and\ to a lesser extent\ potassium "K¦#[ The major anions are chloride "Cl−#\ sulphate "SO1−

3 #\ bicarbonate"HCO−

2 #\ carbonate "CO1−2 #\ and nitrate "NO−

2 #[ These soils are generally divided into three broad categories]saline\ sodic\ and salineÐsodic[ A modi_ed form of the US Salinity Laboratory Sta} "0843# numerical criteriafor these categories\ as de_ned by the Soil Science Society of America "0886#\ is in use in most parts of theworld[ A soil having electrical conductivity of the saturated paste extract "ECe#− 3 dS m−0 and sodiumadsorption ratio "SAR# ³02 is called a saline soil[ Soils having ECe ³ 3 dS m−0 and SAR− 02 are designatedas sodic soils[ If a soil has ECe − 3 dS m−0 and SAR× 02\ it is categorized as a salineÐsodic soil[ SalineÐsodic and sodic soils are generally bracketed together because similar amelioration practices are used forthese soils[

Salt!a}ected soils exist mostly under arid and semiarid regions[ The data regarding the extent and severity

� Correspondence to] Dr M[ Qadir\ Institut fu�r P~anzenerna�hrung\ Interdisziplina�res Forschungszentrum\ Heinrich!Bu}!Ring 15!21\24281 Giessen\ Germany[

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Copyright Þ 1999 John Wiley + Sons\ Ltd[ LAND DEGRADATION + DEVELOPMENT\ 00] 490Ð410 "1999#

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of the salinized area are observational because a practical means of measurement has been lacking untilnow[ However\ estimates show that about 844×095 ha of the world are under di}erent categories of salt!a}ected soils "Szabolcs\ 0883#[ Approximately\ a ratio of 39 ] 59 exists between saline and salineÐsodic:sodicsoils "Tanji\ 0889^ Qadir et al[\ 0885b#[ There has been a temporal increase in magnitude and intensity ofsalt!a}ected soils[ A number of major irrigation schemes throughout the world have su}ered to some extentfrom the e}ects of salinity and:or sodicity[ Many once!productive areas have become salt!a}ected wastelands[Salt!related problems occur within the boundaries of at least 64 countries "Szabolcs\ 0883#[ An alphabeticallist of the countries with serious salinity problems include Australia\ China\ Egypt\ India\ Iraq\ Mexico\Pakistan\ the Soviet Union\ Syria\ Turkey and the United States "Rhoades\ 0887#[ Experts of many inter!national organizations\ including the International Commission on Irrigation and Drainage "ICID#\ theWorld Bank\ the United Nations Environment Program "UNEP#\ and the Food and Agriculture Organ!ization "FAO#\ have reported that the greatest technical cause of declining agricultural productivity onirrigated land\ or irrigation failure\ is salinization and waterlogging of the soil in arid and semiarid regions[Their recommendations\ as summarized by Rhoades "0887#\ focus on urgent measures to be taken to combatdeserti_cation via modi_cation of farming techniques and amelioration of salt!a}ected and waterloggedsoils\ with a resultant improvement in social and economic conditions of people dependent on agriculture[

Soil salinity e}ects are exhibited as reduction in plant growth rate\ reduced yield and\ in severe cases\ totalcrop failure[ In certain circumstances\ excessive salinity in soils cannot be reduced over time by routineirrigation and crop management practices[ Such situations demand amelioration of saline soils for improve!ment of crop productivity and an increase in the number of crop species that can be grown in a speci_carea[ An understanding of saline soil management and amelioration practices is important for long!termsustainable agriculture[ This review provides an analysis of traditional soil salinity management concepts\their limitations and alternatives[ It also focuses on research!based knowledge relating to amelioration ofsaline soils[ It does not cover the extent and causes of irrigation water pollution\ and the use of water withvariable qualities for irrigation[ Extensive reviews on such aspects are available in the literature "Oster\ 0883^Shalhevet\ 0883^ Kijne et al[\ 0887^ Rhoades\ 0887^ Beltra�n\ 0888#[

SOIL SALINITY ASSESSMENT

Traditionally\ soil salinity has been assessed using soil samples collected from the root zone and averagedover time and depth[ It is generally measured via laboratory analysis as electrical conductivity of a saturatedsoil paste extract "ECe#[ Soil paste extracts are soil samples that are brought up to their water saturationpoints "US Salinity Laboratory Sta}\ 0843#[ Electrical conductivities are measured on the vacuum!extractedand _ltered extracts from these samples and are expressed in units of deciSiemens per metre "dS m−0#\ orpreviously as millimhos per centimetre "mmho cm−0#[ Electrical conductivity values reported in both theunits are numerically equal[ Use of saturation extracts as a method of measuring and referencing salinityprovides a direct relationship with the _eld moisture range for most soils[ Soil to water suspensions ofdi}erent ratios such as 0 ] 0\ 0 ] 1\ 0 ] 4 or 0 ] 09 can be more easily made and extracted than obtainingsaturation extracts[ Back calculations can be developed to ECe for a given soil[

Changes in salinity during soil amelioration can be determined from periodic measurements made] "a# onsaturated paste extracts of soil samples^ "b# on extracts of soil to water suspensions of di}erent ratios^ "c# onsoil water samples collected in situ\ usually with vacuum extractors^ "d# in soil\ using buried porous salinitysensors which imbibe and equilibrate with the soil water^ "e# in soil\ using four!electrode probes^ or "f#remotely by an electromagnetic induction technique[ These methods may be used in di}erent situations"Rhoades and Oster\ 0875^ Rhoades\ 0882#[ Among these methods\ determination of salinity changes withthe measurement of bulk soil electrical conductivity "ECa# has been rated as a convenient approach "Rhoades\0887#[ Under conditions of reference soil water content\ ECa is a direct measure of soil salinity[ It is easilyand instantaneously measured in the _eld using either in situ\ four!electrode probes of various types "buriedor portable# or remote!sensing electromagnetic induction devices held by hand above ground "Rhoades and

AMELIORATION OF SALINE SOILS


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Figure 0[ Relation between "a# bulk soil electrical conductivity "ECa# and "b# soil salinity "ECe# and distance along a transect across afurrow!irrigated sugar!beet _eld "Glenbar silty clay!loam soil# located in the Imperial Valley of California\ USA "adapted from

Rhoades\ et al[\ 0886a\ with permission from Elsevier Science#[

Loveday\ 0889#[ Without a requirement of laboratory analysis\ both the salinity assessment techniques havebeen shown to be reliable and practical "Rhoades\ 0882^ Utset et al[\ 0887#[ Data obtained with a mobilefour!electrode sensing system "Figure 0a# show ECa readings collected every second "about 0 m apart# as thetractor moved across a furrow!irrigated sugar beet "Beta vulàris L[# _eld "Rhoades et al[\ 0886a#[ Averageroot zone soil salinities expressed in terms of ECe\ as predicted from the measured ECa data along thetransect and as measured in some calibration samples\ are shown in Figure 0b for these methods[

Geostatistical analysis of a soil salinity data set has been carried out through geographical informationsystems "GIS# on a site of approximately 1499 ha "Bourgault et al[\ 0886#[ Correlations were developedbetween hard "direct measurement of primary variable such as ECe# and soft data that comprised indirectsecondary information related to a primary variable such as vertical electromagnetic "EMV# and horizontalelectromagnetic "EMH# measurements[ Extreme redundancy was found between the two secondary datameasurements "EMV and EMH#\ expressed in units of deciSiemens per metre and representative of the vadosezone "upper 0=1 m soil#[ This redundancy was con_rmed by maps and variograms analysis[ Regarding

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correlation between soft and hard data\ a slightly better correlation "9=62# was found between EMV and ECe

than between EMH and ECe "9=69# for the same soil depth and units of expression[ Again\ the mean valuesof ECe "4=13 dS m−0# and EMV "0=3 dS m−0# had large di}erences[

Di}erences in mean values between hard and soft measurements are not uncommon in the earth sciences[These di}erences can be _ltered out by various unbiasedness constraints in the algorithms used for thesepurposes "Bourgault et al[\ 0886#[ Moreover\ elementary statistics is generally used which ignores datalocations and the relation of data with time and:or space[ Geostatistics propose to add to the GIS toolboxvarious spatial data analysis tools to explore and model patterns of space:time dependence between the dataavailable[ It is suggested that the most robust geostatistical tools be made available to soil scientists andusers of GIS[ There cannot be e.cient data utilization without data interpretation and modelling[ Theresulting numerical models can be used for various mapping purposes and for an assessment of the reliabilityof such maps[ Techniques and guidelines are available elsewhere "Bourgault et al[\ 0886^ Goovaerts\ 0888#[There is a need to raise interest among scientists working in salinity!related aspects to learn more aboutmodern geostatistical concepts of data analysis\ estimation and uncertainty assessment[ Geostatistics mayo}er an increasingly wide palette of techniques well suited to the diversity of the many problems soil scientistshave to deal with[

SALINE SOIL MANAGEMENT] TRADITIONAL CONCEPTS

Salts accumulate in the irrigated root zone when they are left behind as the soil water is used by the growingplants in transpiration or lost by evaporation from the soil surface[ Evapotranspiration creates drying ofthe upper soil relative to deeper layers[ This enforces a potential for an upward ~ow of water into the rootzone[ An understanding of the traditional concepts such as the leaching fraction "LF#\ the salt balance index"SBI#\ and the leaching requirement "LR# is necessary for soil salinity management and amelioration[

To prevent excessive accumulation of salts in the root zone\ as a consequence of irrigation\ extra watermust be applied with irrigation or rainfall should occur in excess of that needed for evapotranspiration[ Thefraction of in_ltrated water that must pass through the root zone to keep soil salinity within an acceptablelevel is referred to as LR[ However\ under actual _eld conditions\ all the extra amount of applied water doesnot pass through the root zone[ Therefore\ the fraction of in_ltrated irrigation that actually percolates belowthe root zone is called LF[ Leaching fraction for a particular site can be calculated from the values of thedepths of irrigation and drainage waters "US Salinity Laboratory Sta}\ 0843#]

LF�Ddw:Diw "0#

where Ddw and Diw are the depths of drainage and irrigation waters\ respectively[ The removal of salts fromthe root zone to maintain the soil solution at a salinity level compatible with the cropping system is referredto as maintaining salt balance[ An SBI may be obtained by calculating the di}erence between the sum ofvarious inputs "Si# and outputs "So# of salts to the soil water salinity "Ssw# of root zone]

DSsw �Si−So "1#

Si �ViwCiw¦VgwCgw¦VrwCrw¦Sm¦Sf "2#

So �VdwCdw¦Sp¦Sc "3#

where ViwCiw �volume and salt concentration of irrigation water that enters the soil^ VgwCgw �volume andsalt concentration of groundwater that reaches the root zone by capillary rise process^ VrwCrw �volume andsalt concentration of rainwater "precipitation# that enters the soil and does not include runo}^ Sm �amountof salts brought into solution from weathering of soil minerals and:or dissolution of salt deposits^ Sf �a!mount of soluble salts added via agricultural chemicals "fertilizers and amendments# and animal manures^VdwCdw �volume and salt concentration of drainage water that moves down as deep percolation^ Sp �a!mount of applied soluble salts in the irrigation water that precipitates in soil after water application^ and



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Sc �amount of salts removed from the soil through the harvested portion of crop that is not added back tothe soil[

Equation "1# may be rewritten by incorporating the individual components of Si and So]

DSsw � "ViwCiw¦VgwCgw¦VrwCrw¦Sm¦Sf#−"VdwCdw¦Sp¦Sc# "4#

Net di}erence between these inputs and outputs results in soil water salinity "DSsw# of root zone[ Understeady!state conditions "DSsw �9#\ assuming] "a# no appreciable contribution of salts from the dissolutionof soil minerals or salts "Sm �9#^ "b# no signi_cant amount of salts added through agricultural chemicalsand manures "Sf �9#^ "c# no loss of soluble salts by precipitation "Sp �9#^ "d# no appreciable removal ofsalts through harvested portion of crop "Sc �9#^ "e# watertable depth su.cient to prevent the introductionof salts into the root zone from capillary rise process "VgwCgw �9#^ "f# no rainfall occurs "VrwCrw �9#^ and"g# uniform aerial application of water on the _eld\ Equation "4# reduces to

9�ViwCiw−VdwCdw "5#

Equation "5# may be rewritten by substituting equivalent depth "D# of water for volume "V#\ and electricalconductivity "EC# for concentration "C#\ since EC of water is a reliable index of its total solute concentrationwithin practical limits "US Salinity Laboratory Sta}\ 0843#]

Ddw:Diw �ECiw:ECdw "6#

Under given steady!state conditions\ Equation "6# shows an approximate equality between leaching fraction"LF�Ddw:Diw# and the ratio of salinity in irrigation and drainage waters "ECiw:ECdw#[ This relationshipshows that LF can also be estimated for these conditions from the values of ECiw and ECdw[ Thus\ it ispossible to control concentration of salts in drainage water within certain limits by varying the fraction ofapplied water that percolates through the root zone[ This may control either the average or the maximumsalinity of the soil in the pro_le at some desired level\ i[e[ intermediate between the ECiw and ECdw levels"Rhoades and Loveday\ 0889#[ Because chloride "Cl−# does not precipitate\ is not adsorbed in the soil andis taken up by the plant roots in small amounts\ the LF can also be calculated as the ratio of Cl− concentrationin irrigation "Cliw# and drainage "Cldw# waters[ Thus\ Equation "6# may be written as

LF�Ddw:Diw �ECiw:ECdw �Cliw:Cldw "7#

Ho}man et al[ "0868# found that the LFs calculated for a number of crops from the Cl− ratio of irrigationand lysimeter drainage waters "LF�Cliw:Cldw# were consistently higher than those based on the measuredwater depth ratio "LF�Ddw:Diw#[ Part of this discrepancy was caused by Cl− uptake by the crops\ but thelargest part of the discrepancy was attributed to the calculation of the weighted average of the Cl−

concentration of drainage water based only on monthly analysis[From the SBI considerations\ it is obvious that some water in excess of evapotranspiration must be applied

with irrigation or rainfall should occur to achieve leaching and to prevent excess salt accumulation in theroot zone[ As already mentioned\ the leaching that prevents excessive salt accumulation in the root zone isreferred to as the LR "US Salinity Laboratory Sta}\ 0843#[ The LR may be achieved either by applyingsu.cient water at each irrigation to meet the LR or by applying\ less frequently\ a leaching irrigationsu.cient to remove the salts accumulated from previous irrigations[ The LR for salinity control may bedetermined using the salt tolerance of the crop "EC?e#\ the salinity of irrigation water "ECiw#\ and the type ofirrigation management "Rhoades et al[\ 0871#]

LR� 9=2975:"F?c#0=691 "8#

LR� 9=0683:"F?c#2=9306 "09#

where F?c is the permissible average concentration factor for root zone "F?c �EC?e:ECiw#[ Equation "8# providesLR values under conventional irrigation in which soil is allowed to dry out between irrigations[ Leaching

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requirement values can be obtained from Equation "09# under high!frequency irrigation in which soil doesnot dry out signi_cantly between irrigations[

There has been an extensive usage of LF\ LR and SBI in soil salinity management and ameliorationprojects[ However\ the preceding considerations suggest that the concept of LR has been used to evaluatethe appropriateness of irrigation and leaching management[ The concept is based on assumptions of steady!state and of absolutely uniform conditions of irrigation\ in_ltration\ leaching and evapotranspiration[ Under_eld conditions\ most of these conditions are not achieved[ The _eld conditions are dynamic and variable\both spatially and temporally[ Salt build!up in the root zone resulting from the presence of a shallowwatertable is ignored in LR calculations[ Moreover\ no practical way exists that directly measures the degreeof leaching being achieved under _eld conditions "Rhoades et al[\ 0886a#[ The SBI concept has been used toevaluate the appropriateness of leaching\ irrigation and drainage practices[ This approach seems inadequatefor these purposes because it provides no information about the absolute level of salinity within the rootzone of any crop or speci_ed _eld in an irrigation project\ and one cannot determine whether or not theirrigation project is tending towards an increase or a decrease in salinity within the root zone[ This is becauseof the fact that salinity from below the soil pro_le and of geologic origin is typically contained in the drainagewater collected from the subsurface drain system "Kaddah and Rhoades\ 0865^ Rhoades et al[\ 0886a#[Additionally\ the transit times involved in the drainage ~ows resulting from a given irrigation event are solong that the SBI values are not re~ective of current trends "Jury\ 0864a\ b#[ These reasons demandincorporation of a rapid and accurate way of monitoring changes in soil salinity during amelioration[

Soil salinity is too variable and transient to be appraised using as large a number of samples that it ispractical to process using conventional soil sampling and laboratory analysis procedures[ It is imperativefor the countries with salinity problems that they should implement assessment programmes which providethis information in a timely and e.cient way[ In many developing countries\ lack of reliable information onthe nature and extent of salt!a}ected areas has been considered as a major contributing factor to the slowprogress of soil salinity management and amelioration projects[ The periodic collection of information onsoil salinity level and distribution within the root zone of an irrigated project is possible to obtain and usefulto inventory conditions of soil salinity[ This may help in assessment of the adequacy of leaching and drainage\and may guide management and amelioration practices[ Such information can also be used to delineate thedi}use sources of salt loading within irrigated lands and to map the distribution and extent of drainageproblem areas[ Techniques and guidelines are described elsewhere "Rhoades et al[\ 0888#[

SALINE SOIL AMELIORATION] GENERAL CONSIDERATION

Amelioration of saline soils can be accomplished using a number of methods[ Selection of an appropriatesite!speci_c amelioration method depends on a number of soil properties and other considerations such asthe depth of soil to be ameliorated\ presence of compacted layer"s# in the subsoil\ the type and content ofsalts present\ the quality and quantity of water available for leaching\ the quality and depth of groundwater\the topographic features of land\ the nature of the crop"s# to be grown after amelioration\ the climaticconditions and the time available for amelioration[

Adequate soil drainage is considered as an essential prerequisite for successful soil amelioration[ Naturalinternal drainage alone may be adequate if there is su.cient storage capacity in the soil pro_le or a permeablesubsurface layer occurs that drains to a suitable outlet[ An arti_cial system must be provided if such naturaldrainage is not present[ Otherwise the resultant soil amelioration will not be sustainable[ Besides adequatesoil drainage\ land levelling and deep groundwater are also basic components of saline soil amelioration[The suitable depth of groundwater depends on climate\ groundwater quality\ and crop"s# to be grown[ Apartfrom control of soil salinity\ the _nal result of soil amelioration must be a su.cient and stable porosity thatprovides an environment physically favourable for water movement\ and for the growth and developmentof plant roots[

Chemical reactions in the root zone of an irrigated soil a}ect soil salinity and sodicity and salt load of



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drainage water[ Signi_cant reactions include dissolution and precipitation of minerals\ adsorption anddesorption of CO1 gas and boron "B#\ and formation of SO1−

3 \ CO1−2 \ and HCO−

2 ion pairs in the solutionphase "Oster and Rhoades\ 0889#[ Dutt and Tanji "0851# and Dutt "0851# were the _rst to demonstrate theuse of computer programs to predict the inorganic compositions of soil solutions[ Oster and McNeal "0860#evaluated their usefulness in calculating changes in the soil solution composition with changing soil watercontent[ Numerous models have been developed in an attempt to quantify the physical and chemicalprocesses that a}ect movement and reactions of salts in soils "Dutt et al[\ 0861^ Tanji et al[\ 0861^ Jury et al[\0868^ Oster and Frenkel\ 0879^ van Genuchten\ 0879^ Robbins et al[\ 0879a\ b^ Bresler et al[\ 0871^ Schulzand Reardan\ 0872^ Feddes et al[\ 0877^ Simunek and Suarez\ 0883\ 0886^ Suarez and Simunek\ 0886^ amongothers#[ Considerable temporal\ theoretical improvement has been obtained in developing models of di}erenttypes[ However\ evaluation of these models under _eld conditions is very limited[ Robbins et al[ "0879b#tested their model by comparing its results with experimental data obtained from a lysimeter study[ However\in a _eld study\ Dudley et al[ "0870# indicated that the obbins et al[ "0879b# model could predict salinity\ butnot speci_c ion concentrations[ Most models of salt reactions and transport in soils do not appropriatelydescribe large variations in multiple concurrent processes that mostly occur under actual _eld conditions"Jury\ 0873^ Rhoades and Loveday\ 0889^ Oster et al[\ 0885^ Kelleners et al[\ 0888#[ However\ attempts viarecent modelling "Simunek and Suarez\ 0883\ 0886^ Suarez and Simunek\ 0886# have been made to overcomethe de_ciency of a suitable model that can quantitatively integrate and describe mathematically most of theprocesses operating during salt movement through the soil pro_le[ For example\ the Suarez and Simunek"0886# model contains several features including expressions relating reductions in hydraulic conductivity tochemical factors\ prediction of CO1 partial pressure in the root zone based on a CO1 productionÐmultiphasetransport submodel\ kinetic expressions for silicate weathering\ calcite precipitation dissolution and dolomitedissolution\ representation of B adsorption using the constant capacitance model\ prediction of cationexchange selectivity\ and a plant growth submodel that includes water\ salinity and oxygen stress[ Again\the recently developed SuarezÐSimunek models need evaluation by _eld studies[ Therefore\ predictionsbased on transport theory are not being widely used for amelioration of salt!a}ected soils[ Consequently\amelioration guidelines are usually made on the basis of experimental relations derived from _eld andlysimeter experiments[

SOIL AMELIORATION STRATEGIES

A number of methods have been used for the amelioration of saline soils[ These methods include] "a#movement of excess soluble salts from upper to lower soil depths via leaching\ which may be accomplishedby continuous ponding\ intermittent ponding\ or sprinkling^ "b# surface ~ushing of salts from soils thatcontain salt crusts at surface\ a shallow watertable or a highly impermeable pro_le^ "c# biological reductionof salts by harvest of high!salt accumulating aerial plant parts\ in areas with negligible irrigation water orrainfall available for leaching^ and "d# amelioration of saline soils under cropping in conjunction withleaching[

Leachin` of Saline Soils

Leaching of salts from the top soil to lower depths down below the root zone is a prevalent method of salinesoil amelioration[ It is applicable to those areas where su.cient water is available to enforce a net downwardmovement of soil solution through the root zone[ Salt leaching interacts closely with irrigation method\irrigation water quality\ soil solution composition\ salt tolerance of crop"s# to be grown after amelioration\composition and depth of groundwater\ and soil hydraulic properties "van Schilfgaarde\ 0865^ Rhoades\0871^ Keren and Miyamoto\ 0889^ Rhoades\ 0882^ Beltra�n\ 0888#[ Leaching controls salt accumulation inthe soil\ provides drainage water and in~uences drainage requirements and drainage water quality[

Magnitude of leaching needed to ameliorate saline soils can be either measured directly in the _eld orestimated through some indirect methods[ The actual amount of leaching required for amelioration of a

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particular saline soil may be determined through water application by a suitable method followed bydetermination of the changes in salinity of the soil pro_le[ The progress of salt removal via leaching can beimmediately observed from the ECa readings[ This simply establishes the rate of reduction in salinity withwater application and time and\ hence\ describes the magnitude of leaching needed for a particular _eld[

Under ideal soil conditions\ where there is no pore bypass\ dissolution of precipitated salts\ salt di}usionconstraints or dispersion\ the soil solution concentration passing through a given depth in the soil pro_leshould decrease to the concentration of the applied water after the passage of one pore volume "the portionof soil bulk volume occupied by soil pores# of applied water[ However\ soils seldom behave in such an idealway "Oster et al[\ 0885#[ Considerable bypass may occur during leaching\ particularly in structured soilswhere water moves preferentially through interpedal macropores relative to micropores within the peds "vanHoorn\ 0870^ McIntyre et al[\ 0871^ Rhoades et al[\ 0886b#[ The extent of the bypass\ in general\ is directlyproportional to the clay content\ rate of leaching\ content of precipitated salts in the pro_le and content ofsoil water maintained during leaching[

Since amelioration of saline soils depends on the movement of water through the soil pro_le to removeexcess salts from the root zone\ it is important that leaching and drainage for salinity control should "a#minimize ~ow of water through the soil pro_le to reduce dissolution of soil minerals and "b# reduce drainagevolume[ Reduction in drainage volume may be achieved through water economy and by placing drains atcertain depths in such a way that the spatial and temporal variability of soil water and soil salinity distributionremain favourable for optimum crop returns\ and only unavoidable water is collected from shallow aquifers"Gupta and Abrol\ 0889#[

Successful desalinization of saline soils needs a downward movement of the zone of salt accumulation toa depth from where resalinization is not possible or extremely limited[ In the case of shallow groundwater\particularly in areas of seepage or faulty drainage\ the zone of salt accumulation hardly moves downwards[This may lead to the formation of salineÐwaterlogged soils[ If the groundwater is deep\ as in the case ofmany natural saline soils of arid and semiarid regions\ the zone of salt accumulation moves down to aconsiderable depth[ In this situation\ su.cient irrigation water must be applied or rainfall should occurabove the evaporation need of a fallow _eld or evapotranspiration requirement of a cropped area[ Theexcess water passes through and beyond the root zone to carry away the salts with it[ Thus\ leaching preventsexcessive salt accumulation in the root zone[ A number of leaching methods have been used for theamelioration of saline soils[ These include continuous ponding\ intermittent ponding and sprinkling[

Continuous pondin`

Typically\ about 69 per cent or more of the soluble salts initially present in a saline soil pro_le can beremoved by leaching with a depth of water equivalent to the depth of soil to be ameliorated when water isponded continuously on the soil surface "Ho}man\ 0875#[ This guideline is more applicable to medium!textured soils[ In terms of pore volume\ 0=4Ð1=9 pore volume of water must pass through the soil to cause adecrease in salt concentration by this amount "Rhoades and Loveday\ 0889#[ Ho}man "0875# summarizedthe results of a number of _eld trials that were conducted in various parts of the world "Figure 1#[ Heproposed the following relationship between the fraction of initial salt concentration remaining in the pro_leand the amount of water in_ltrating through the pro_le per unit depth of soil\ when water was pondedcontinuously on the soil surface]

"C:C9#"Dw:Ds#�K? "00#

where C:C9 is the fraction of the initial salt concentration remaining in the soil pro_le\ Dw is the depth ofwater in_ltrated\ Ds is the depth of the soil to be leached\ and K? is a constant that di}ers with soil type[Equation "00# de_nes the curves in Figure 1 for organic "peat# soils when K?�9=34\ for _ne!textured "clay!loam# soils when K?�9=2 and for coarse!textured "sandy loam# soils when K?�9=0[ The data in Figure 1for sandy loams are from two experiments in India "Le}elaar and Sharma\ 0866^ Khosla et al[\ 0868# andone in Iraq "Hulsbos and Boumans\ 0859#[ The data for clay!loams are from three experiments in Utah on



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Figure 1[ Depth of leaching water per unit depth of soil required to reclaim saline soils by continuous ponding "adapted from Ho}man\0875\ with permission from Springer!Verlag#[

a clay!loam\ a silty clay!loam\ and a clay "Reeve et al[\ 0837#\ and from two experiments in California on aclay!loam "Reeve et al[\ 0844# and a silty clay "Oster et al[\ 0861#[ The data on peat soils are from Turkey"Beyce\ 0861#\ with a _eld experiment conducted in California "Prichard et al[\ 0874# providing additionaldata that support the peat soil relationship[ Equation "00# does not account for evaporation losses[ Conse!quently\ where evaporation is 09 per cent of the depth of leaching water applied "9=0Dw#\ Dw should becorrected for evaporation "Minhas and Khosla\ 0875#[ The equation is valid only when Dw:Ds exceeds K?[The values of K? re~ect di}erences in saturated volumetric content and leaching e.ciency among soils[Sandy loam soils with low!saturated!water contents have higher leaching e.ciencies than _ne!textured soils[Soil pores in sandy soils are more uniform in diameter than in clay!loam or clay soils[ Fine!textured soilscan have large cracks with large pores between aggregates and along crack surfaces when dry\ and have _nepores within aggregates when wet[ Such bypass channels reduce leaching e.ciency of _ne!textured soils"Oster et al[\ 0885^ Rhoades et al[\ 0886b#[

Intermittent pondin`

The amount of water required for leaching soluble salts can be reduced by intermittent applications ofponded water\ particularly for _ne!textured soils[ Compared to continuous ponding\ intermittent pondingmay reduce by about one!third the volume of water required to achieve the same degree of salt removal\ i[e[69 per cent of soluble salts "Ho}man\ 0875#[ However\ the rate of soil amelioration by intermittent pondingmay be slower than continuous ponding[ In a comparative study involving continuous and intermittentponding methods "Miller et al[\ 0854#\ about 49 per cent more water was required for amelioration withcontinuous ponding[ But the time required for leaching a clay!loam soil was much longer for intermittent

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Figure 2[ Depth of leaching water per unit depth of soil required to reclaim saline soils by ponding water intermittently "adapted fromHo}man\ 0875\ with permission from Springer!Verlag#[

ponding[ However\ Oster et al[ "0861# achieved the same amelioration in three months by both the methods^the saline soil under reclamation was _ner in texture "silty clay# than that used by Miller et al[ "0854#[ Theresults of three leaching experiments by intermittent ponding "Figure 2# have shown an excellent agreementconsidering that soil texture ranged from silty clay "Oster et al[\ 0861# to loam and sand "Talsma\ 0856# andthat the depth of water applied to each cycle varied from 49 to 049 mm\ with corresponding ponding intervalsvarying from weekly to monthly[ Therefore\ for leaching by intermittent ponding\ K? in Equation "00# is nothighly soil texture!dependent[ Its value can be approximated for a wide range of soil texture classes as

"C:C9#"Dw:Ds#� 9=0 "01#

Intermittent ponding has a particular importance in areas with a shallow watertable or a tile drainagesystem[ It allows the watertable to draw down\ which increases leaching e.ciency "Talsma\ 0856#[ In soilsthat develop surface seal\ it helps water to in_ltrate by forming cracks[ However\ the longer period of wetsoil exposure increases evaporation[ A combination of intermittent ponding and mulching was foundhelpful in improving performance of intermittent ponding "Carter and Fanning\ 0853#[ This combination isimportant in areas having low rates of drainage and evaporation along with a high watertable[ However\Minhas and Khosla "0875# have demonstrated that intermittent ponding does not lead to any saving inwater for leaching of salts under high temperature conditions which have high evaporative demand[

Sprinklin`

Amelioration of saline soils can be achieved through sprinkling under continuously unsaturated conditionsif water application rates are controlled so that ponding does not occur[ In sprinkler irrigation\ solute



400

transport is governed by the combined processes of convection "movement of solutes with the bulk solution#and di}usion "independent movement of solutes as driven by a concentration gradient#[ Convection isusually the predominant process in ~ood!irrigated soils[ Di}erential velocities of water ~ow can occur withinthe soil matrix because pore!size distribution is typically non!uniform[ In the case of high ~ow velocity andlarge pore!size distribution\ di}usion often limits salt removal from saline soils[ Much of the water and saltspresent in small intraaggregate pores are bypassed in ~ood!irrigated soils[ Flow velocity and water contentare typically lower in sprinkler!irrigated soils[ Thus\ sprinkler irrigation evades the ine.cient leaching causedby bypass ~ow in large cracks developed in _ne!textured soils "Rhoades\ 0887#[

The range of leaching is con_ned to the depth of soil wet by sprinkler irrigation[ Leaching e.ciency ofsprinkler irrigation may be similar to that of intermittent ponding[ The e.ciency may be improved where alow irrigation rate is required or intermittent sprinkling is practised\ particularly in areas with a largedrainage load[ If sprinkler irrigation occurs more frequently than intermittent ponding\ the time!averagedwater content within the depth wet by sprinkling can be higher than under intermittent ponding[ Under _eldconditions\ Oster et al[ "0861# found the counteracting e}ects of reduced bypass ~ow and increased watercontent consistent on a silty clay soil with the observed order of leaching e.ciency] intermittent pon!ding× sprinkling× continuous ponding[ Since measurements of soil salinity and water application havebeen reported on only a few leaching experiments\ therefore\ generalization is di.cult[

Crops irrigated with sprinkler irrigation may be subject to injury not only from salts in the soil but alsofrom salts absorbed directly through wetted leaf surfaces "Maas\ 0874#[ Saline water should not be used asan irrigation source via sprinkling on susceptible crops because their foliage absorbs salts upon wetting[ Saltaccumulation in leaves by foliar absorption of such crops could become lethal[ Crop sensitivity to salinesprinkling water is related more to the rate of foliar salt accumulation than to crop tolerance to soil salinityper se "Maas\ 0875#[ Frequency of irrigation also in~uences the onset of salt injury "Francžois and Clark\0868^ Maas et al[\ 0871#[ Infrequent\ heavy irrigations may be applied rather than frequent\ light irrigations[Slowly rotated sprinklers that allow drying between cycles should be avoided\ since this practice increasesthe wettingÐdrying frequency "Francžois and Maas\ 0888#[

Weather conditions play a crucial role in foliar salt accumulation[ For example\ salt concentrations thatcause severe leaf injury and necrosis after one or two days of hot\ dry and windy weather may not cause anysymptoms under cool and humid weather conditions[ Alternatively\ applications should be made during thenight when evaporation is less[ Hot\ dry and windy days should be avoided[ Surface!applied irrigation"s#may substitute sprinkler irrigation"s# if irrigation is inevitable under these conditions[ These practices canminimize the foliar absorption of salts\ particularly under conditions when susceptible crops are sprinkler!irrigated with saline water[ Numerous other factors can a}ect the amount of salts accumulated by leaves\including leaf age\ shape\ angle and position on plant\ type and concentration of salt\ presence of a waxycuticular layer on the leaf surface\ ambient temperature\ irrigation frequency and length of time the leafremains wet "Francžois and Maas\ 0888#[ Sprinkler irrigation is bene_cial when the _eld is not prepared forleaching under ponded conditions[ It is compatible with unlevelled sandy soils\ particularly when water forleaching is scarce and costly[ On the other hand\ it requires high energy and capital[ Besides\ careful watermanagement is an integral part of sprinkler irrigation[

Surface Flushin` of Saline Soils

Some saline soils may contain visible crusts of salts at the surface\ a shallow watertable or a highlyimpermeable layer near to the surface[ These situations have led to attempting to ameliorate such soils bysurface ~ushing\ i[e[ passing water over the soil surface and wasting the runo} water to be collected at asloppy end of the _eld[ Reeve et al[ "0844# found that passing water over the surface by sheet ~ow failed toremove a signi_cant amount of salts[ Flushing removed less than 9=0 per cent of the salts in the top 9=2 m ofa silty clay!loam soil[ In a lysimeter experiment "Yadav and Agarwal\ 0848#\ an increased amount of saltremoval from a salineÐsodic soil was obtained when the lysimeters were given ~ushings between leachingswhen compared with leaching alone[ Recently\ Qadir et al[ "0887# evaluated surface ~ushing as an alternative

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to leaching for amelioration of a low!permeability salineÐsodic _eld[ It involved mixing of applied waterwith the surface soil by a cultivator followed by ~ushing the standing water from the soil surface to a nearbydrain and a second similar ~ushing repeated after 01 hours "FF#[ In some plots\ gypsum was applied betweenthe two ~ushings "FGF#[ The FF treatment could remove only 3 per cent of the salts present in the top 9=2m of the soil[ Salt removal by the FGF treatment from the same depth of soil was about 6 per cent[ However\the FGF treatment was able to remove an appreciable quantity of exchangeable Na¦ as a result of Ca1¦ÐNa¦ exchange[

Two prerequisites have been identi_ed for surface ~ushing of salt!a}ected soils under _eld conditions\ i[e[provision of enough slope gradient in the land\ and the presence of a nearby drain to collect the ~ushed saltywater "Agarwal et al[\ 0868^ Qadir et al[\ 0887#[ Apart from its ine.cient amelioration output\ surface~ushing of saline soils seems to be a cost!intensive as well as an uneconomical approach[ Physical removalof surface salt crusts by mechanical means may be an alternative[

Harvest of Hi`h!Salt Accumulatin` Aerial Plant Parts

Saline soil amelioration has been attempted through biological reduction of salts that consisted of removalof high!salt accumulating aerial plant parts[ This approach was used by Chaudhri et al[ "0853# in areas withnegligible irrigation water or rainfall available for leaching[ They used sea!blithe ðSuaeda fruiticosa "L[#ForskŁ for salt extraction from soil[ This species\ which has a very low irrigation requirement\ is among thefew plants that can grow in highly saline soils[ Thus\ sea!blithe has been found suitable for the area borderingthe desert which is frequented by dust storms and has little rainfall[ It was estimated that about 1399 lb"0977 kg# of salts could be removed from one acre "³1=6 Mg ha−0# by a single harvest of the aerial parts ofthe plant species in the autumn each year[

Under the prevalent irrigated agriculture\ salts are being continuously added to soils with each irrigation[Removal of salts via aerial plant parts seems an ine.cient way of saline soil amelioration[ For example\ asalt!tolerant perennial forage crop Leptochloa fusca "L[# Kunth "Booth\ 0872# or previously Diphlachne fusca

"L[# Beauv\ commonly known as Kallar\ Karnal or Australian grass\ is grown during amelioration of salt!a}ected soils in many parts of the world[ The species is rated as a potential biotic material for soil amelioration"Kumar and Abrol\ 0873^ Sandhu and Qureshi\ 0875^ Qadir et al[\ 0885b^ Kumar\ 0885#[ Its above!groundpart "forage# contains salt in the range of 39Ð79 g kg−0 when grown in soils with ECe values of about 19 dSm−0 "Malik et al[\ 0875#[ Annual forage production of the grass ranges from 19 to 29 Mg ha−0 "Kumar et

al[\ 0883^ Qadir et al[\ 0885b#[ Numerical evaluation of salt removal through aerial parts of the grass is givenin Table I by using] "a# irrigation waters of di}erent salinities "ECiw �9=4\ 0=9 and 1=9 dS m−0#^ "b# forageyield values of 19\ 14\ and 29 Mg ha−0^ "c# salt concentration in forage ranging from 39 to 79 g kg−0^ and"d# volume of irrigation water used during its growth period as 9=7 ha m−0 "7×095 litres#[ Using these values\

it can be estimated that the species is not even capable of removing all the salt added through irrigation

water used for its growth[ The projected calculations indicate that under ideal conditions of maximum forage

yield "29 Mg ha−0#\ highest salt content in forage "79 g kg−0#\ and use of good!quality irrigation water

"ECiw �9=4 dS m−0#\ the grass can remove about 89 per cent of the salt added through irrigation water[

However\ under actual _eld conditions where high electrolyte waters "ECiw × 0=4 dS m−0# are used for

irrigation of salt tolerant crops\ foraging part of the grass can hardly remove up to 19 per cent of the salt

added via irrigation[ In the case of less salt!tolerant crops than Kallar grass\ salt removal through aerial

plant parts could be even smaller than these estimates[ An additional consideration deals with the fact that

these calculations have been done while keeping a complete oversight on ambient soil salinity levels[

With respect to salt removal through forage as the sole contributing factor in controlling soil salinity

under irrigated conditions\ there seems to be an increase in soil salinity rather than a decrease in it[ However\

this increase is not observed when saline soils are put to amelioration by growing salt!tolerant crops[ A

decrease in soil salinity is generally observed in saline soils under cropping and leaching[ The amount of salt

removed through forage harvest is small when compared to the salt content of the soil or that of irrigation



402

Table I[ Numerical evaluation of salt removal through aerial parts of a salt tolerant grass ðLeptochloa fusca "L[# KunthŁusing] "a# irrigation waters of di}erent salinities "ECiw � 9=4\ 0=9 and 1=9 dS m−0#^ "b# forage yield values of 19\ 14 and29 Mg ha−0^ "c# salt concentration in forage in the range of 39 to 79 g kg−0^ and "d# volume of irrigation water usedduring its growth period as 9=7 ha m−0 "7×095 litres#

ECiw "dS m−0#Forage yield"Mg ha−0# 9=4 0=9 1=9

Salt removal$ "Mg ha−0# through forage having salt concentration of 39 g kg−0

19 9=7 "29=7#% 9=7 "04=3# 9=7 "6=6#14 0=9 "27=4# 0=9 "08=1# 0=9 "8=5#29 0=1 "35=1# 0=1 "12=0# 0=1 "00=4#

Salt removal$ "Mg ha−0# through forage having salt concentration of 79 g kg−0

19 0=5 "50=4#% 0=5 "29=7# 0=5 "04=3#14 1=9 "65=8# 1=9 "27=4# 1=9 "08=1#29 1=3 "81=2# 1=3 "35=1# 1=3 "12=0#

Salt added& "Mg ha−0# through application of 9=7 ha m−0 "7×095 litres# irrigation water1=5 4=1 09=3

$ Calculated as] ð"forage yield in Mg ha−0# "forage salt concentration in g kg−0#Ł:092[% Figures in parenthesis indicate percentage removal of salt added by respective irrigation[& Calculated as] ð"ECiw in dS m−0# "539# "7×095#Ł:098[

water used to grow the crop[ Therefore\ the principal source of soil salinity decrease in irrigated areas seemsto be leaching of salts out of the root zone rather than removal of salts by aerial plant parts[

Croppin` in Conjunction with Leachin`

Since it has been established that salt removal through aerial parts of agronomic crops is insigni_cant\leaching is also necessary for the amelioration of saline soils under cropped conditions[ An increase in salt!leaching e.ciency may often be observed under copping and irrigation by reducing the soil water contentmaintained during leaching[ This is because of the fact that unsaturated ~ow conditions cause a decrease inlarge pore bypass "Dahiya et al[\ 0870^ Oster et al[\ 0885#[ Bene_cial e}ects of plant roots during soilamelioration include] "a# the physical action of plant roots improves the soil structure and provides channelsfor in_ltrating water^ "b# an increase in soil organic matter via contribution from below!ground plantmaterial that consists of roots^ and "c# an increase in the dissolution of lime "CaCO2# in the presence of CO1

evolved from root respiration and decomposition of organic matter "Chhabra and Abrol\ 0866^ Robbins\0875a\ b^ Boyle et al[\ 0878^ Oster et al[\ 0885^ Qadir et al[\ 0885a#[ Ahmad et al[ "0889# found an increase insalt!leaching e.ciency of a cropped salineÐsodic _eld than under a non!cropped control treatment[ Thecropped and non!cropped plots received approximately the same quantity of irrigation water[ However\leaching e.ciency was variable among the crops\ being in the order] sesbania "Sesbania aculeata Per!s[#×Kallar grass× sordan ðSor`hum bicolor "L[# Moench×Sor`hum sudanese "Piper# StapfŁ[ Sesbaniacaused a more uniform decrease in the soil pro_le salinity\ owing to its extensive root system that providedchannels for the percolating soil solution[ In other _eld experiments\ a similar trend of soil ameliorationwith cropped and non!cropped treatments has been observed "Singh and Singh\ 0878^ Qadir et al[\ 0885b#[An added advantage relates to better availability of some macro! and micronutrients after soil ameliorationthat involved cropping and leaching compared to leaching alone "Qadir et al[\ 0886#[

Apart from the root e}ect\ the aerial plant portion also contributes towards a decrease in soil salinity[This portion provides shade to the soil\ lowers soil temperature and decreases evaporation from the soilsurface compared to a non!cropped surface[ In a _eld experiment "Qadir et al[\ 0885b#\ forage productionwas found directly proportional to the relative decrease in soluble salts of a salineÐsodic _eld "pHs �7=3Ð

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7=7\ ECe �8=5Ð00=9 dS m−0\ SAR�48=3Ð61=3#[ Sesbania produced forage 21=2 Mg ha−0\ followed byKallar grass at 13=5 Mg ha−0\ millet rice ðEchinochloa colona "L[# LinkŁ at 11=5 Mg ha−0\ and _nger milletðEluesine coracana "L[# GaertnŁ at 4=3 Mg ha−0[ The amelioration e}ect of the plant species regarding thereduction in soil ECe and SAR followed the same order[

Although cropping duration amelioration of saline soils is a common practice\ only a few crops cantolerate high ambient salinity levels\ particularly during early stages of crop growth[ High seedbed salinitycauses a delay in seed germination and reduces plant vigour during seedling establishment[ Maintenance ofadequate moisture is also di.cult when additional irrigation is needed or heavy rainfall occurs during aprolonged period of seed germination and plant establishment[ Therefore\ seedbed salinity should becontrolled with respect to the threshold salinity level of the crop "Maas and Ho}man\ 0866^ Francžois andMaas\ 0888^ Katerji et al[\ 1999# to obtain good seed germination and crop establishment[ Leaching beforeplanting any crop should be done if initial soil salinity is high "ECe × 09Ð04 dS m−0# in the upper soil layer"Oster et al[\ 0885#[

CROP SELECTION FOR SALINE SOILS

Selection of plant species capable of satisfactory biomass production during soil amelioration is vital[Generally\ high water!requirement crops get the bene_t of leaching resulting from a greater number ofirrigations\ while salt!tolerant crops enjoy the facility of their natural as well as adaptive mechanisms whenplanted in a saline environment[ Therefore\ the crops that are able to colonize salt!a}ected soils are importantfor soil amelioration[

Plants growing in a saline environment generally face certain limitations[ Soil salinity may reduce cropyields by upsetting the water and nutritional balance of plants[ Selection of crops during and after ameli!oration of saline soils should be based on their respective salt tolerances "Bernstein\ 0864^ Maas and Ho}man\0866^ Maas\ 0875^ Marcum\ 0888^ Grattan and Grieve\ 0888^ Francžois and Maas\ 0888^ Katerji et al[\ 1999#[The salt tolerance of a crop is not an exact value[ It re~ects the ability of a crop to resist the adverse\ non!speci_c e}ects of excessive root zone salinity[ Although the capacity of a crop to endure salinity cannot bestated in absolute terms\ the relative crop responses to known salinities under certain conditions can bepredicted[ Maas and Ho}man "0866# proposed a linear response model to characterize crops regarding theirsalt tolerances[ Two parameters obtained from this model are] "a# the threshold soil salinity "the maximumallowable soil salinity for a crop without yield reduction#^ and "b# the slope "the percentage yield decreaseper unit decrease in salinity beyond the threshold salinity level#[ The data\ presented in terms of ECe at14 >C\ serve only as a guideline to relative tolerances among crops[ Absolute tolerances may vary and dependon climate\ soil condition and cultural practices "Table II#[ The threshold and slope values obtained fromthe MaasÐHo}man model can be used to calculate relative yield "Yr# for any given soil salinity exceedingthe threshold level by using the following equation]

Yr � 099−s"ECe−ECt# "02#

where ECt � threshold salinity level expressed in dS m−0^ s�slope expressed in per cent per dS m−0^ andECe �average electrical conductivity of the saturated soil paste extract of the root zone expressed as dSm−0[The tolerances of crops to salinity as presented in Figure 3 are generally divided into four classes\ i[e[sensitive\ moderately sensitive\ moderately tolerant\ and tolerant "Maas and Ho}man\ 0866^ Francžois andMaas\ 0888#[ Generally\ the threshold and linear slope for a crop remain within one class[ Where the linearcurve for a crop crosses division boundaries\ the crop was classi_ed based on the tolerance at lower levels atwhich yield was commercially acceptable[

Salt tolerances of a number of grain\ forage\ _bre\ fruit and vegetable crops are given in Table II\ whichis adapted from Francžois and Maas "0888#[ Some crops are listed with only a qualitative rating because thedata are insu.cient to calculate the threshold salinity level and slope[ Salt tolerances of these crops range



404

Table II[ Salt tolerances of some grain\ _bre\ forage\ fruit and vegetable crops as a function of root zone salinity$"adapted from Francžois and Maas\ 0888\ with permission from Marcel!Dekker#

Crop Salt tolerance parameters

Tolerance Threshold& SlopeCommon name Botanical name based on ECe "dS m−0# ") per dS m−0# Rating%

Fibre\ grain and special cropsBarley' Hordeum vulàre L[ Grain yield 7=9 4=9 T$$Bean Phaseolus vulàris L[ Seeds or pods 0=9 08=9 SCanola Brassica campestris L[ Seed yield Ð Ð T

ðsyn[ Brassica rapa L[ŁCanola Brassica napus L[ Seed yield Ð Ð TChannel millet Echinochloa tumerana Grain yield Ð Ð T

"Domin# J[ M[ BlackChickpea Cicer arietinum L[ Seed yield Ð Ð MSCorn Zea mays L[ Ear fresh weight 0=6 01=9 MSCotton Gossypium hirsutum L[ Seed cotton yield 6=6 4=1 TDurum wheat Triticum turìdum L[ var[ Grain yield 4=8 2=7 T

drumu Desf[Oats Avena sativa L[ Grain yield Ð Ð TPeanut Arachis hypoàea L[ Seed yield 2=1 18=9 MSRice%% Oryza sativa L[ Grain yield 2=9 01=9 SRye Secale cereale L[ Grain yield 00[3 09=7 TSa/ower Carthamus tinctorius L[ Seed yield Ð Ð MTSorghum Sor`hum bicolor "L[# Moench Grain yield 5=7 05=9 MTSoybean Glycine max "L[# Merrill Seed yield 4=9 19=9 MTSugar beet&& Beta vulàris L[ Storage root 6=9 4=8 TSugar cane Saccharum of_cinarum L[ Shoot dry weight 0=6 4=8 MSSun~ower Helianthus annuus L[ Seed yield Ð Ð MTTriticale ×Triticosecale Wittmack Grain yield 5=0 1=4 TWheat Triticum aestivum L[ Grain yield 5=9 6=0 MT

Grasses and forage cropsAlfalfa Medicaò sativa L[ Shoot dry weight 1=9 6=2 MSBermuda grass Cynodon dactylon "L[# Pers[ Shoot dry weight 5=8 5=3 TBerseem clover Trifolium alexandrinum L[ Shoot dry weight 0=4 4=6 MSBroad bean Vicia faba L[ Shoot dry weight 0=5 8=5 MSDesert salt grass Distichlis spicta L[ var[ Shoot dry weight Ð Ð T�

stricta "Torr[# BettleDurum wheat Triticum turìdum L[ var[ Shoot dry weight 1=0 1=4 MT

durum Desf[Foxtail millet Setaria italica "L[# Beauvois Dry matter Ð Ð MSGuinea grass Panicum maximum Jaq[ Shoot dry weight Ð Ð MTHarding grass Phalaris tuberosa L[ var[ Shoot dry weight 3=5 6=5 MT

stenoptera "Hack#A[ S[ Hitchc[

Kallar grass Leptochloa fusca "L[# Shoot dry weight Ð Ð TKunth ðformerly Diplachnefusca Beauv[Ł

Meadow fescue Festuca pratensis Huds[ Shoot dry weight Ð Ð MT�Narrowleaf birdsfoot Lotus corniculatus var[ Shoot dry weight 4=9 09=9 MTtrefoil tenuifolium L[Oats Avena sativa L[ Straw dry weight Ð Ð TOrchard grass Dactylis `lomerata L[ Shoot dry weight 0=4 5=1 MS

continued overleaf

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Table II "continued#



Persian clover Trifolium resupinatum L[ Shoot dry weight Ð Ð MS�Red clover Trifolium pratense L[ Shoot dry weight 0=4 01=9 MSReed canary grass Phalaris arundinacea L[ Shoot dry weight Ð Ð MTRhodes grass Chloris àyana Kunth[ Shoot dry weight Ð Ð MTRye grass\ perennial Lolium perenne L[ Shoot dry weight 4=5 6=5 MTSweet clover Melilotus sp[ Mill[ Shoot dry weight Ð Ð MT�Sesbania Sesbania exaltata "Raf[# Shoot dry weight 1=2 6=9 MS

V[ L[ CorySesbania Sesbania bispinosa "Linn[# Shoot dry weight Ð Ð MT

W[ F[ Wright ðsyn[ Sesbaniaaculeata "Willd[# PoirŁ

Sudan grass Sor`hum sudanese "Piper# Shoot dry weight 1=7 3=2 MTStapf

Tall fescue Festuca elatior L[ Shoot dry weight 2=8 4=2 MTTall wheat grass A`ropyron elonàtum Shoot dry weight 6=4 3=1 T

"Hort# Beauvois

Vegetables and fruit cropsAsparagus Asparaùs of_cinalis L[ Spear yield 3=0 1=9 TBean Phaseolus vulàris L[ Seed yield 0=9 08=9 SCabbage Brassica oleracea L[ Head fresh weight 0=7 8=6 MS

"capitata group#Carrot Daucus carota L[ Storage root 0=9 03=9 SCauli~ower Brassica oleracea L[ Ð Ð MS�

"botrytis group#Celery Apium `raveolens L[ var[ Petiole fresh 0=7 5=1 MS

dulce "Mill[# Pers[ weightCowpea Vi`na unùiculata "L[# Walp[ Seed yield 3=8 01=9 MTCucumber Cucumis sativus L[ Fruit yield 1=4 02=9 MSEggplant Solanum melonèna L[ Fruit yield 0=0 5=8 MS

var[ esculentum Nees[Garlic Allium sativum L[ Bulb yield 0=6 09=9 MSLettuce Lactuca sativa L[ Top fresh weight 0=2 02=9 MSMung bean Vi`na radiata "L[# R[ Wilcz[ Seed yield 0=7 19=6 SMuskmelon Cucumis melo L[ "reticulatus Fruit yield 0=9 7=3 MS

group#Okra Abelmoschus esculentus "L[# Pod yield Ð Ð MS

MoenchOnion "bulb# Allium cepa L[ Bulb yield 0=1 05=9 SOnion "seed# Allium cepa L[ Seed yield 0=9 7=9 MSPea Pisum sativum L[ Seed fresh weight 2=3 09=5 MSPepper Capsicum annuum L[ Fruit yield 0=4 03=9 MSPotato Solanum tuberosum L[ Tuber yield 0=6 01=9 MSPurslane Portulaca oleracea L[ Shoot fresh weight 5=2 8=5 MTRadish Raphanus sativus L[ Storage root 0=1 02=9 MSSpinach Spinacia oleracea L[ Top fresh weight 1=9 6=5 MSStrawberry Fraària×Ananassa Duch[ Fruit yield 0=9 22=9 SSweet potato Ipomoea batatas "L[# Lam[ Fleshy root 0=4 00=9 MSTomato Lycopersicon lycopersicum Fruit yield 1=4 8=8 MS

"L[# Karst[ ex Farw[

continued



406

Table II "continued#



Turnip Brassica rapa L[ "rapifera Storage root 9=8 8=9 MSgroup#

Zucchini squash Cucrubita pepo L[ var[ Fruit yield 3=6 8=3 MTmelopepo "L[# Alef[

$ These data serve only as a guideline to relative tolerances among crops[ Absolute tolerances vary depending on climate\ soil conditionsand cultural practices[% Ratings are de_ned by the boundaries given in Figure 3[ Ratings marked by asterisk are estimates[& In gypsiferous soils\ plants tolerate ECe about 1 dS m−0 higher than indicated[' Less tolerant during seedling stage\ ECe at this stage should not exceed 3 or 4 dS m−0[$$ Salt tolerance ratings abbreviated as T "tolerant#\ MT "moderately tolerant#\ MS "moderately sensitive# and S "sensitive#[%% Because rice is grown under ~ooded conditions\ values refer to the electrical conductivity of the soil water while the plants aresubmerged[ Less tolerant during seedling stage[&& Sensitive during germination and emergence\ ECe should not exceed 2 dS m−0[

Figure 3[ Divisions for classifying crop tolerances to salinity "adapted from Francžois and Maas\ 0888 with permission from Marcel!Dekker#[

from sensitive to tolerant[ This genetic diversity provides many options for growing a range of crops duringand after soil amelioration[ Genotypic variability also occurs in many crops such as sorghum ðSor`hum

bicolor "L[# MoenchŁ "Grieve and Maas\ 0877#\ rice "Oryza sativa L[# "Yeo and Flowers\ 0872^ Grieve andFujiyama\ 0876^ Aslam et al[\ 0882#\ triticale "×Triticosecale# "Norlyn and Epstein\ 0873#\ wheat "Triticum

aestivum L[# "Kingsbury and Epstein\ 0875#\ sesbania "Yasin et al[\ 0889# cotton "Gossypium hirsutum L[#"Khan\ 0876^ Qadir and Shams\ 0886#\ alfalfa "Medicaò sativa L[# "Rogers et al[\ 0887#\ and many vegetables"Shannon and Grieve\ 0888#\ among others[ The wide inter! and intracrop diversity regarding crop salttolerance "Shannon\ 0886# suggests that _eld trials be conducted to identify local crops that are adaptableto saline soil conditions[

Crops used as an amelioration tool for saline soils may experience oxygen de_ciency when excessive

M[ QADIR ET AL[


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irrigations are applied to leach salts from top soil to lower depths[ Root zone salinity in conjunction withoxygen de_ciency greatly increases salt uptake compared with saline non!waterlogged conditions "Akhtaret al[\ 0883#[ This e}ect may cause failure in active transport and exclusion processes in the root membrane"West\ 0867^ Drew\ 0872^ West and Taylor\ 0873#[ There seems to be a need to evaluate salt!tolerant cropsfor their resistance again hypoxia[ The genotypes showing greater tolerances against the combined e}ects ofsalinity and hypoxia may be a better choice for soil amelioration[ Thus\ genotypes of increased tolerancesto salinity and hypoxia should be sought or developed through genetics and bioengineering processes[ Useof suitable rotations of the salinityÐhypoxia!tolerant crops during soil amelioration may help promotelowering of the watertable\ leaching of salts and soil aggregation[

CONCLUSIONS

Excessive salt accumulation in soils is one of the most serious problems agriculture is facing in arid andsemiarid regions[ The e}ects of salinity are expressed through an overall decline in yield and\ in severecases\ total crop failure[ An understanding of saline soil amelioration options is important for sustainableagriculture on such soils[ Among the various methods used\ leaching of salts from upper to lower soil depthshas been a prevalent method of saline soil amelioration[ Keeping aside some temporary relief\ indiscriminateuse of excessive leachings and consequently high drainage volumes have generally resulted in an overall landdegradation by aggravated salinization and waterlogging[ However\ in most irrigation projects\ the currentlyused leaching levels can be reduced with an improvement in irrigation management without harming cropsor soils[ Alternatively\ cropping during leaching or between leachings may be done to increase salt leachinge.ciency because a decrease in soil water content occurs under unsaturated water ~ow conditions with aconcurrent decrease in drainage volume[ However\ crop genotypes of increased tolerances to salinity andhypoxia should be sought or developed\ which can produce useful biomass on saline soils[ Use of suitablerotations of the salinityÐhypoxia tolerant crops during soil amelioration may help promote lowering of thewatertable\ leaching of salts and soil aggregation[

Evaluation of traditional concepts such as LF\ SBI and LR suggests that saline soil amelioration strategiesneed more exact inventories regarding level\ extent and distribution pattern of root zone salinity viamonitoring and assessment procedures[ These procedures should be simple\ rapid\ e.cient and economical[Four!electrode probe "Rhoades\ 0882^ Utset et al[ 0887# and:or electromagnetic induction "Rhoades\ 0882\0888# methods could be used as an alternative to routine ECe determination for soil salinity appraisal[However\ there is a need to raise interest among scientists working in salinity related aspects to learn moreabout modern geostatistical concepts of data analysis\ estimation and uncertainty assessment[ Geostatisticsmay o}er an increasingly wide palette of techniques well suited to the diversity of many problems soilscientists have to deal with[

Numerous models have been developed that provide considerable insight in understanding movementand reactions of salts in soils[ Considerable theoretical improvement in modelling has been obtained"Simunek and Suarez\ 0886^ Suarez and Simunek\ 0886# by incorporating di}erent processes that occur insalt!a}ected soils[ However\ evaluation of these models is needed by _eld studies that may result in theirextensive use for soil amelioration[

ACKNOWLEDGEMENTS

We are grateful to Drs M[ C[ Shannon\ J[ D[ Rhoades and D[ L[ Suarez of the George E[ Brown JrSalinity Laboratory\ Riverside\ and Professor J[ D[ Oster\ Department of Soil and Environmental Sciences\University of California\ Riverside\ CA\ for providing many reports and reprints of original papers thathelped in the preparation of this paper[



408

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M[ QADIR ET AL[


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https://www.researchgate.net/publication/248883654_Leaching_of_tile-drained_saline_soils?el=1_x_8&enrichId=rgreq-f8a922c3a681121511870aca818d35d8-XXX&enrichSource=Y292ZXJQYWdlOzI0Nzk5MTc4NztBUzoxODMyNDY4MDU1NDQ5NjBAMTQyMDcwMDg1MTIwMg==

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Amelioration strategies for saline soils: a review

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